Inorg. Chem. 2002, 41, 1534−1544
Ruthenium−Manganese Complexes for Artificial Photosynthesis: Factors Controlling Intramolecular Electron Transfer and Excited-State Quenching Reactions Malin L. A. Abrahamsson,† Helena Berglund Baudin,† Anh Tran,‡ Christian Philouze,‡ Katja E. Berg,‡ Mary Katherine Raymond-Johansson,† Licheng Sun,‡ Bjo1 rn A° kermark,‡ Stenbjo1 rn Styring,§ and Leif Hammarstro1 m*,† Department of Physical Chemistry, Uppsala UniVersity, Box 532, S-751 21 Uppsala, Sweden, Department of Organic Chemistry, Stockholm UniVersity, S-106 91 Stockholm, Sweden, and Department of Biochemistry, Center for Chemistry and Chemical Engineering, UniVersity of Lund, Box 124, S-221 00 Lund, Sweden Received July 9, 2001
Continuing our work toward a system mimicking the electron-transfer steps from manganese to P680+ in photosystem II (PS II), we report a series of ruthenium(II)−manganese(II) complexes that display intramolecular electron transfer from manganese(II) to photooxidized ruthenium(III). The electron-transfer rate constant (kET) values span a large range, 1 × 105−2 × 107 s-1, and we have investigated different factors that are responsible for the variation. The reorganization energies determined experimentally (λ ) 1.5−2.0 eV) are larger than expected for solvent reorganization in complexes of similar size in polar solvents (typically λ ≈ 1.0 eV). This result indicates that the inner reorganization energy is relatively large and, consequently, that at moderate driving force values manganese complexes are not fast donors. Both the type of manganese ligand and the link between the two metals are shown to be of great importance to the electron-transfer rate. In contrast, we show that the quenching of the excited state of the ruthenium(II) moiety by manganese(II) in this series of complexes mainly depends on the distance between the metals. However, by synthetically modifying the sensitizer so that the lowest metal-to-ligand charge transfer state was localized on the nonbridging ruthenium(II) ligands, we could reduce the quenching rate constant in one complex by a factor of 700 without changing the bridging ligand. Still, the manganese(II)−ruthenium(III) electrontransfer rate constant was not reduced. Consequently, the modification resulted in a complex with very favorable properties.
Introduction In nature, plants efficiently convert light energy emitted from the sun into chemical energy by the process known as photosynthesis.1,2 When energy from the sun reaches photosystem II (PS II), one of the two reaction centers in oxygenevolving organisms, the photoactive P680 chlorophylls are excited. An electron is then transferred from the excited P680 to a primary acceptor and then to the reaction-center quinones, creating a charge-separated state, the energy of * Corresponding author. E-mail:
[email protected]. † Uppsala University. ‡ Stockholm University. § University of Lund. (1) The Photosynthetic Reaction Center; Deisenhofer, J., Norris, J.R., Eds.; Academic Press: San Diego, 1993; Vols. 1 and 2. (2) Barber, J.; Andersson, B. Nature 1994, 370, 31.
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which is used in the photosynthetic reactions. The oxidized photosensitizer, P680+, is reduced to its original oxidation state through an intricate series of reactions involving the oxygenevolving manganese cluster and a tyrosine residue, tyrosineZ, a redox-active intermediate positioned between P680 and the manganese cluster.3-9 The cluster stores up to four oxidizing equivalents, resulting in the catalytic oxidation of two water (3) Debus, R. J. Biochim. Biophys. Acta 1992, 1102, 269. (4) Barber, J.; Nield, J.; Morris, E. P.; Zheleva, D.; Hankamer, B. Physiol. Plant. 1997, 100, 817. (5) Debus, R. J. Manganese and Its Role in Biological Processes; Marcel Dekker: Basel, Swizerland, 2000. (6) Yachandra, V. K.; Sauer, K.; Klein, M. P. Chem. ReV. 1996, 96, 2927. (7) Ru¨ttinger, W.; Dismukes, G. C. Chem. ReV. 1997, 97, 1. (8) Britt, R. D. In Oxygenic Photosynthesis: The Light Reactions; Ort, D., Yocum, C., Eds.; Kluwer Academic Publishers: Dortrecht, The Netherlands, 1996; p 137.
10.1021/ic0107227 CCC: $22.00
© 2002 American Chemical Society Published on Web 02/22/2002
Complexes for Artificial Photosynthesis
molecules. Four electrons, four protons, and one molecule of oxygen are produced in each catalytic cycle, after which the manganese cluster is restored to its most reduced state. During the last 30 years, much effort has been devoted to the construction of an artificial system that mimics the natural way of converting solar energy to chemical energy. Several model systems have been constructed (e.g., mimics of the primary charge-separation processes 10-14 and manganese complexes serving as models for the oxygen-evolving center in PS II6,7). Many attempts have been made to synthesize manganese complexes capable of catalytic water oxidation, but so far the success has been modest.7,15-17 Recently, Zouni et al.18 presented the crystal structure of PS II at 3.8 Å resolution, including the manganese cluster. This structure is a major achievement that together with additional, more refined structures will give important information about the manganese cluster. This information will play an important role in the development of manganese complexes that mimic the oxygen-evolving reaction. Our work has been focused on mimicking the whole donor side of PS II by synthesizing a supramolecular system containing both a manganese moiety and a ruthenium(II) trisbipyridyl moiety as the photooxidizable sensitizer.19-22 In this contribution, we present a series of ruthenium(II)manganese(II) complexes with different types of manganesebinding ligands and links between the metal centers. The observed rate constants for intramolecular electron transfer from manganese(II) to photooxidized ruthenium(III) vary from 1 × 105-2 × 107 s-1. Because the goal is to create a system where regeneration of the primary donor ruthenium(II) is fast and efficient, it is important to understand what (9) Diner, B. A.; Babcock, G. T. In Oxygenic Photosynthesis: The Light Reactions; Ort, D., Yocum, C., Eds.; Kluwer Academic Publishers: Dortrecht, The Netherlands, 1996; pp 213-247. (10) Kurreck, H.; Huber, M. Angew. Chem., Int. Ed. Engl. 1995, 34, 849. (11) Sauvage, J.-P.; Collin, J.-P.; Chambron, J.-C.; Guillerez, S.; Coudret, C.; Balzani, V.; Barigelletti, F.; De Cola, L.; Flamigni, L. Chem. ReV. 1994, 94, 993. (12) Gust, D.; Moore, T. A.; Moore, A. L. In Electron Transfer in Chemistry: Biological and Artificial Supramolecular Systems; Balzani, V., Ed.; Wiley-VCH: Weinheim, Germany, 2001; Vol. 3, pp 272336. (13) Wasielewski, M. R. Chem. ReV. 1992, 92, 435. (14) Scandola, F.; Chiorboli, C.; Indelli, M. T.; Rampi, M. A. In Electron Transfer in Chemistry: Biological and Artificial Supramolecular Systems; Balzani, V., Ed.; Wiley-VCH: Weinheim, Germany, 2001; Vol. 3, pp 337-408. (15) Manchanda, R.; Brudwig, G. W.; Crabtree, R. H. Coord. Chem. ReV. 1995, 144, 1. (16) Pecoraro, V. L.; Baldwin, J. M.; Gelasco, A. Chem. ReV. 1994, 94, 807. (17) Limburg, J.; Vrettos, J. S.; Liable-Sands, L. M.; Rheingold, A. L.; Crabtree, R. H.; Brudwig, G. W. Science 1999, 283, 1524. (18) Zouni, A.; Witt, H. T.; Kern, J.; Fromme, P.; Krauss, N.; Saenger, W.; Orth, P. Nature 2001, 409, 739-743. (19) Sun, L.; Berglund, H.; Davydov, R.; Norrby, T.; Hammarstro¨m, L.; Korall, P.; Bo¨rje, A.; Philouze, C.; Berg, K.; Tran, A.; Andersson, M.; Stenhagen, G.; Mårtensson, J.; Almgern, M.; Styring, S.; A° kermark, B. J. Am. Chem. Soc. 1997, 119, 6996. (20) Sun, L.; Hammarstro¨m, L.; Norrby, T.; Berglund, H.; Davydov, R.; Andersson, M.; Bo¨rje, A.; Korall, P.; Philouze, C.; Almgren, M.; Styring, S.; A° kermark, B. Chem. Commun. 1997, 607. (21) Berglund Baudin, H.; Sun, L.; Davidov, R.; Sundahl, M.; Styring, S.; A° kermark, B.; Almgren, M.; Hammarstro¨m, L. J. Phys. Chem. A 1998, 102, 2512. (22) Berg, K. E.; Tran, A.; Raymond, M. K.; Abrahamsson, M.; Wolny, J.; Redon, S.; Andersson, M.; Sun, L.; Styring, S.; Hammarstro¨m, L.; Toftlund, H.; A° kermark, B. Eur. J. Inorg. Chem. 2001, 2001, 1019.
could cause this variation. We have therefore investigated different factors that affect the electron-transfer rate. These investigations will provide us with information to be used in the construction of more complicated systems containing several manganese ions that are capable of multielectron transfer.23-25 We have also reported that the manganese(II) quenches the ruthenium(II) excited state in some complexes, presumably by intramolecular energy transfer.21,22 In the present contribution, we discuss this reaction that competes with the desired photooxidation and present data for a larger series of complexes, showing that the quenching rate constant falls off exponentially with the metal-to-metal distance. However, by localizing the ruthenium-based metal-to-ligand charge-transfer excited state on the remote ligands, the quenching rate constant could be reduced by 2 orders of magnitude. Experimental Section Synthesis and Characterization. The synthesis and characterization of the mononuclear ruthenium complexes 1a-5a and the binuclear ruthenium(II)-manganese(II) complexes 1b-5b are described elsewhere.19,22 All the 1H NMR spectra described below were measured either on Brucker-400 MHz or Brucker-500 MHz spectrometers. Ru(bpy)2(4-CH3-4′-(N′-CH3-N,N′-Bis(2-pyridylmethyl)-1,2ethanediamine)-(CH2)5-bpy)(PF6)2 (6a). This complex was prepared according to the literature procedure described by Sun et al.19 4-Methyl-4′-pentyl-(5′′-N(N′-methyl-N,N′-bis(2-pyridylmethyl)-1,2ethane-diamine))-2,2′-bipyridine (6, bpy-(CH2)5-bispicen) was synthesized by reaction of 4-methyl-4′-(5′′-pentylbromo)-2,2′-bipyridine with N-methyl-N,N′-bis(2-pyridylmethyl)-1,2-ethanediamine(bispicen). The ligand was then purified by column chromatography on aluminum oxide using CH2Cl2 and CH2Cl2/MeOH (98:2) as eluents. 1H NMR (400 MHz, in CDCl3): δ 1.36-1.78 (m, 6H, -CH2CH2CH2-), 2.28 (s, 3H, N-CH3), 2.50 (s, 3H, bpy-CH3), 2.55 (t, J ) 5.5 Hz, 2H, -CH2CH2CH2-N), 2.67 (t, J ) 5.4 Hz, 2H, CH3-N-CH2CH2-N-), 2.80 (t, J ) 5.4 Hz, 2H, CH3-NCH2CH2-N-), 2.81-2.83 (m, 2H, bpy-CH2CH2CH2-), 3.68 (s, 2H, Py-CH2-N), 3.84 (s, 2H, Py′-CH2-N), 7.10-7.29 (m, 4H, bpy-H and Py-H), 7.48-7.56 (m, 2H, bpy-H and Py-H), 7.657.78 (m, 2H, bpy-H and Py-H), 8.12 (s, 1H, bpy-H), 8.308.35 (m, 2H, bpy-H and Py-H), 8.52 (s, 1H, bpy-H), 8.518.62 (m, 2H, bpy-H and Py-H). To obtain the ruthenium compound (6a), an equivalent amount of cis-Ru(bpy)2Cl2‚2H2O was added to a methanol solution of the bpy-(CH2)5-bispicen ligand (6) while stirring the solution under argon atmosphere. The brown solution obtained was then heated and refluxed for one week, resulting in a clear orange solution. After evaporation of the solvent, a red solid residue was obtained. The residue was purified by MPLC on neutral aluminum oxide, and the eluents used in the gradient were CH2Cl2 and CH2Cl2/MeOH (94:6). Dissolving the product in a minimum amount of methanol followed by addition of a concentrated NH4PF6 solution gave a red precipitate of 6a. 1H NMR (400 MHz, DMSO-d6): δ 1.341.79 (m, 6H, -CH2CH2CH2-), 2.14 (s, 3H, N-CH3), 2.18 (m, (23) Sun, L.; Raymond, M. K.; Magnuson, A.; LeGourrie´rec, D.; Tamm, M.; Arahamsson, M.; Kenez, H.; Mårtensson, J.; Stenhagen, G.; Hammarstro¨m, L.; Styring, S.; A° kermark, B. J. Inorg. Biochem. 2000, 78, 15. (24) Burdinski, D.; Wieghardt, K.; Steenken, S. J. Am. Chem. Soc. 1999, 121, 10781. (25) Burdinski, D.; Bothe, E.; Wieghardt, K. Inorg. Chem. 2000, 39, 105.
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Abrahamsson et al. 2H, -CH2Cl2CH2-N), 2.20 (m, 2H, bpy-CH2CH2CH2-), 2.45 (s, 3H, bpy-CH3), 2.80-2.90 (m, 4H, CH3-N-CH2CH2-N-), 3.68 (s, 2H, Py-CH2-N), 3.80 (s, 2H, Py′-CH2-N), 7.25-8.86 (m, 30H, bpy-H and Py-H). Ru(bpy)2(4-CH3-4′-(N′-CH3-N,N′-Bis(2-pyridylmethyl)-1,2ethanediamine)-(CH2)5-bpy)Mn(PF6)2Cl2 (6b). To a methanol solution of 6a a solution of MnCl2‚4H2O was added while stirring at room temperature, immediately giving a red precipitate. The reaction mixture was stirred for another 20 min, and the red precipitate was filtered off and washed with a small amount of cold methanol followed by diethyl ether. The red precipitate was then dried in air to give 6b as a red solid in 66% yield. 1H NMR (400 MHz, DMSO-d6): δ values were similar to those of 6a, but all peaks were broadened because of the paramagnetic manganese(II). 4,4′-Bis(ethoxycarbonyl)-2,2′-bipyridine (8). 4,4′-Dicarboxy2,2′-bipyridine26 (2.0 g, 8.2 mmol) in 12 mL of thionyl chloride was refluxed overnight. Excess thionyl chloride was evaporated under reduced pressure. The resulting solid was suspended in 5 mL of ethanol (99.5%), and 0.5 mL of Et3N was added. This solution was refluxed for 12 h under nitrogen. The mixture was evaporated to dryness, and the resulting solid was suspended in 20 mL of water to dissolve the Et3N‚HCl salt that formed during reaction. The diester, 8, was then collected by suction filtration, washed well with water, and dried to yield a white powder (2.3 g, 92%). 1H NMR (400 MHz, CDCl3): δ 1.45 (t, J ) 7.2 Hz, 6H, OCH2CH3), 4.46 (q, J ) 7.2 Hz, 4H, OCH2CH3), 7.91 (dd, J ) 5.2 Hz, 1.6 Hz, 2H, bpy-H), 8.87 (d, J ) 5.2 Hz, 2H, bpy-H), 8.95 (d, J ) 1.6 Hz, 2H, bpy-H). Ru[bpy(CO2Et)2]2[Cl]2 (9). cis-RuCl2(DMSO)427 (0.85 g, 1.8 mmol) was suspended in 30 mL of ethylene glycol. The suspension was heated at 150 °C for a few minutes until the solid had dissolved, and then LiCl (1.1 g, 26 mmol) was added. When all the LiCl salt had dissolved (a few minutes), the diester (8) (1.0 g, 3.3 mmol) was added in small portions over 1 h. The reaction mixture was heated for another 20 min and then cooled in an ice bath. Fifty milliliters of water was added, and the precipitate was collected by suction filtration and washed with water and a mixture of acetone/diethyl ether (1:2). A brown powder was obtained (0.45 g, 33%). 1H NMR (400 MHz, DMSO-d6): δ 1.29 (t, J ) 7.2 Hz, 6H, OCH2CH3), 1.43 (t, J ) 7.2 Hz, 6H, OCH2CH3), 4.34 (q, J ) 7.2 Hz, 4H, OCH2CH3), 4.51 (q, J ) 7.2 Hz, 4H, OCH2CH3), 7.48 (dd, J ) 5.6, 1.6 Hz, 2H, bpy-H), 7.74 (d, J ) 5.6 Hz, 2H, bpyH), 8.24 (dd, J ) 6.0 Hz, 1.6 Hz, 2H, bpy-H), 8.93 (d, J ) 1.6 Hz, 2H, bpy-H), 8.11 (d, J ) 1.6 Hz, 2H, bpy-H), 10.95 (d, J ) 6.0 Hz, 2H, bpy-H). Ru[bpy(CO2Et)2]2[4′-Phenyl(4′′-CH2-bis(2-pyridylmethyl))bpy][PF6]2 (7a). A solution of 9 (0.079 g, 0.10 mmol) and silver triflate (0.053 g, 0.20 mmol) in 10 mL of acetone was heated at 70 °C for 17 h under a nitrogen atmosphere. The solution was allowed to cool to ambient temperature and then filtered to remove the AgCl salt before 4′-(p-methylbromophenyl)bipyridine22 (0.033 g, 0.10 mmol) was added. This mixture was heated for 6 h under nitrogen and then evaporated to dryness. The red-brown residue was dissolved in a minimum amount of methanol, and a saturated solution of NH4PF6 (10 mL) was added to precipitate the trisbipyridyl ruthenium complex. The precipitate was collected by filtration and washed with water and Et2O. It was then dissolved in CH2Cl2 (20 mL), and dipicolylamine28 (0.020 g, 0.10 mmol) (26) Garelli, N.; Vierling, P. J. Org. Chem. 1992, 57, 3046. (27) Evans, I. P.; Spencer, A.; Wilkinson, G. J. Chem. Soc., Dalton Trans. 1973, 204.
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and Et3N (0.010 g, 0.10 mmol) were added. The solution was heated at 45 °C for 13 h and then evaporated to dryness and purified by column chromatography, first using CH3CN and then using 8:1:1 CH3CN/H2O/KNO3(saturated) as eluents. A red fraction eluted after a brown band was collected. After evaporation of the solvent, the solid was redissolved in a minimum amount of methanol, and a solution of NH4PF6 was added. The red precipitate was collected by filtration, washed with water and Et2O, and dried to give 0.066 g (46%) of the desired complex. 1H NMR (400 MHz, acetone-d6): δ 1.38 (t, J ) 7.2 Hz, 12H, OCH2CH3), 3.87 (s, 6H, PhCH2N and NCH2Py), 4.47 (q, J ) 7.2 Hz, 8H, OCH2CH3), 7.26-7.30 (m, 2H, Py-H), 7.59-7.63 (m, 1H, bpy-H), 7.65 (d, J ) 7.7 Hz, 2H, Py-H), 7.71 (d, J ) 8.1 Hz, 2H, Ph-H), 7.78-7.80 (m, 2H PyH), 7.86 (dd, J ) 5.9 Hz, 1.8 Hz, 1H, bpy-H′), 7.93 (d, J ) 8.1 Hz, 2H, Ph-H), 7.94-7.96 (m, 2H, bpy-ester-H), 7.99-8.03 (m, 2H, bpy-ester-H), 8.12 (d, J ) 5.9 Hz, 1H, bpy-H′), 8.138.15 (m, 1H, bpy-H), 8.29 (dt, J ) 7.7 Hz, 1.5 Hz, 1H, bpy-H), 8.36-8.43 (m, 4H, bpy-ester-H), 8.55-8.57 (m, 2H, Py-H), 9.14-9.17 (m, 2H, bpy-H and bpy-H′), 9.34-9.36 (m, 4H, bpyester-H). ESI-MS m/z: (M-PF6) 1290.08, (M-2PF6) 572.69. Anal. Calcd for C61H57N9O8F12P2Ru‚0.5CH2Cl2: C, 49.99, H, 3.96, N, 8.53. Found: C, 49.78, H, 4.02, N, 8.52. 7b was synthesized by adding a saturated solution of MnCl2 in acetonitrile to 7a dissolved in acetonitrile. ESI-MS of 7b (prepared as described previously) m/z: (M - PF6 - solvent) 1415.18, (M - 2PF6 - solvent) 635.14. Luminescence Spectroscopy. The steady-state emission and absorption measurements were performed at room temperature in air-saturated acetonitrile of spectroscopic grade (Merck, 99.8%). The absorption spectra were recorded on an HP 8453 diode array spectrophotometer, and the emission spectra were recorded using a SPEX fluorolog II system. The excited-state lifetimes of all the complexes were determined in nitrogen-purged acetonitrile using a time-correlated single-photon counting setup described previously.29 Low-temperature measurements were performed in butyronitrile (Fluka, 99%) using capillary tubes inserted into a coldfinger Dewar filled with liquid nitrogen. Electrochemistry. The electrochemistry of 1a-3a, 5a, 7a, and 1b-5b was performed in an argon-filled glovebox. The electrolyte was 0.1 M tetrabutylammonium tetrafluoroborate (TBABF4) in acetonitrile (Aldrich, 99.8%), which was used as received. The salt was dried at 140 °C for 48 h before preparing the electrolyte. For 2b, the temperature dependence of the redox potentials was also investigated between 10 and 60 °C. A three-electrode system consisting of Ag/AgCl in LiCl-saturated acetonitrile as the reference electrode, a platinum wire or a carbon stick as a counter electrode, and a freshly polished piece of glassy carbon (diameter 3 mm) as the working electrode were used. Both the reference electrode and the counter electrode were separated from the solution by a salt bridge that was in contact with the working electrode. A potentiostat from Eco Chemie with an Autolab/GPES electrochemical interface was used. The reported half-wave potentials, E1/2 (E1/2 ) (Ep,a + Ep,c)/2), which were measured versus ferrocenium/ferrocene as an internal reference, are reported versus SCE. The conversion was made by setting the RuIII/II value obtained for Ru(bpy)3(Cl)2 equal to 1.324 V vs SCE.30 (28) Larsen, S.; Michelsen, K.; Pedersen, E. Acta Chem. Scand. 1986, A 40, 63. (29) Almgren, M.; Hansson, P.; Mukhtar, E.; van Stam, J. Langmuir 1992, 8, 2405. (30) Tokel-Takvoryan, N. E.; Hemingway, R. E.; Bard, A. J. J. Am. Chem. Soc. 1973, 95, 6582.
Complexes for Artificial Photosynthesis Scheme 2
Scheme 1
Electron Transfer. Electron transfer in 1a-6a and 1b-6b was studied with laser flash photolysis as described elsewhere.31 For 7a and 7b, the transient absorption was studied using a flashphotolysis system with a Q-switched Nd-YAG laser (λ ) 355 nm) to pump an OPO delivering
